1. Field
The present invention relates to triple-clad optical fibers and devices utilizing triple-clad optical fibers and, more specifically, to triple-clad optical fibers having an inner cladding with a high numerical aperture and endoscopes utilizing such triple-clad optical fibers.
2. Technical Background
Endoscopes, specifically non-linear optical endoscopes utilizing two-photon fluorescence processes have emerged as one of the best, non-invasive means of performing fluorescence microscopy on a sample and/or a subject. The non-linear optical endoscopes utilize a source or excitation laser beam directed onto a target with an optical fiber. The interaction of the excitation laser beam with the target causes the target material to fluoresce or emit light due to two photon absorption, which is collected back into the endoscope with the optical fiber for further analysis, imaging, spectroscopy and the like. Compared with single-photon microscopy, two-photon fluorescence microscopy offers inherent optical sectioning properties, greater penetration depths and flexible optical spectra accessibility.
One of the key components of non-linear optical endoscopy utilizing the two-photon fluorescence process is the optical fiber employed in the endoscope. While standard single mode optical fiber may be utilized in the endoscope to deliver the excitation laser beam to the target of interest, these optical fibers are not suitable for collecting the optical signals emitted by the target back into endoscope for further analysis, image formation, spectroscopy and the like. This is primarily due to the low numerical aperture and small core size of standard single mode fibers which limits the collection efficiency of the optical fibers.
To improve the collection efficiency of the non-linear optical endoscope, double-clad optical fibers may be used. Such optical fibers generally include a core, an inner cladding, and an outer cladding. The excitation beam of the endoscope is delivered to the target via the core of the optical fiber and the light emitted from the target is collect back into the endoscope via the inner cladding of the optical fiber. To have high collection efficiency, a high numerical aperture of the inner cladding is desired. The numerical aperture can be increased by doping the inner cladding with an index raising dopant. In this case, the core has to be up-doped with a higher doping level (Ge or Al) to create a waveguide in the core. However doping the fiber core of the double-clad fiber with high concentration of dopant (while having an up-doped inner cladding) will increase the background noise. In endoscope applications, it is desirable to collect the two-photon signal with low noise background. To reduce the background noise, one can use a double clad fiber that has a core with low up-dopant levels, and the inner cladding that is pure silica or doped with an index decreasing dopant, to create a waveguiding structure. However, the lower refractive index in the inner cladding reduces the NA of the inner cladding, which decreases the collection efficiency of the double clad fiber. Thus one of skill in the art, in designing double-clad fibers for endoscope applications, typically has to emphasize either a good collection efficiency or a low background noise.
Therefore, there is a need to design a fiber with low dopant levels in the core but with high numerical aperture in the cladding to increase the collection efficiency in the cladding. Accordingly a need exists for alternative optical fibers and non-linear optical endoscopes employing the same.